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heat-resistant steels (namely, steam pipeline steels operated mainly under static loads, and
blade steels operated mainly under cyclic loads) are substantiated as a start of merging of
damages located along separate (single) grain boundaries with formation of damages sur-
rounding grain conglomerates.
The criterion for assessing the structural and mechanical state of the 15Kh1M1F steel
after long-term operation is justified by the deviation from linearity of the dependence be-
tween its grain sizes and hardness values (Hall-Patch type) determined by measurements
on the pipe surface. It is shown that for the same duration of steel operation on the main
steam pipelines, only the steel has reached the critical state which had undergone a greater
number of unit shut-downs.
The methodology for assessing the structural and mechanical state of a number of
structural steels after their operation on various objects has been scientifically substantiat-
ed basing on the plotted unified correlations between the drops of their mechanical charac-
teristics and the change in informative indicators of the structural and fractographic chang-
es in steels due to their degradation after long-term operation in both ambient and high-
temperature conditions.
Keywords: degradation, structural steels, long-term operation, ambient and high-
temperature conditions, correlations, fractographic and mechanical indicators, critical state
substantiation.